In recent years, regarding to television broadcast, digital ground wave broadcast service and digital CATV broadcast service have been provided with the advances of digital compression technique and digital modulation/demodulation technique. In these broadcast services, video data is coded by MPEG2 coding system, and the digital modulation system is employed for the transmission of the coded bit stream. Particularly, 8VSB modulation system is employed for the terrestrial broadcasting in North America, and QAM system is employed in digital CATV system, respectively, being co-used with conventional NTSC analog broadcasting.
An example of conventional multi-system corresponding receiver which can correspond to the digital broadcasting and the analogue broadcasting will be described with reference to the drawings.
FIG. 9 is a block diagram of a conventional multi-system corresponding receiver which can correspond to ground wave broadcasting, NTSC analogue broadcasting, and digital cable broadcasting.
A construction of a portion corresponding to the digital ground wave broadcasting receiver will be first described.
In this receiver, there are provided a tuner 91 for selecting a channel of RF modulated-wave signal which is input from a terminal 90; a SAW (Surface Acoustic Wave) filter 92 for applying a band limitation to the selected modulated-wave signal; an AMP 93 for amplifying a modulated-wave signal; an orthogonal detector circuit 94 for detecting the modulated wave using a mixer; a low pass filter (LPF) 95 for cutting high frequency components of the detected signal; a digital VSB demodulation circuit 96 for performing 8VSB demodulation; and a digital decoding and video processing circuit 97 for MPEG decoding and video signal processing.
The operation of the digital ground wave broadcasting 8VSB receiver constituted as above will now be explained. The modulated wave tuned by the tuner 91 is output as an IF signal to the SAW filter 92. After being limited to a specified frequency characteristic by the SAW filter 92, the demodulated wave is amplified by the AMP 93 and is input to the orthogonal detector circuit 94.
FIG. 10 shows an 8VSB digital modulated wave which is represented in a frequency region. As shown in FIG. 10, the 8VSB digital modulated wave is transmitted at a frequency band width of 6 MHz which is the same as that in analog NTSC broadcasting, and a pilot signal fp is added.
Accordingly, the synchronous wave-detection with frequency synchronization and phase synchronization is performed to the pilot signal to be converted into the 8VSB base band signal. The 8VSB signal dropped into the base band signal is cut for its high frequency signals by the LPF 95 to be outputted to the VSB demodulation circuit 96, decoded by the demodulation circuit 96, and converted into the transport data. The transport data is MPEG decoded and video signal processed by the digital decoding and video processing circuit 97 to become an analogue video signal to be outputted to a terminal 121, and displayed on such as a monitor via the switch 122 and the video signal output terminal 124.
Meanwhile, the structure corresponding to the analogue broadcasting receiver includes a tuner 98 for channel selecting an input RF analog modulated signal; an AMP 99 for amplifying the selected modulated signal; a Nyquist filter 100 having a symmetric slope characteristic against the carrier frequency; an orthogonal detector circuit 111 for performing synchronous detection of the carrier wave; a LPF 112 for applying band limitation to the detected signal; and an analog video signal processing part 113.
The operation of the analogue NTSC broadcasting receiver will now be described. An analog RF modulated wave which is input to the tuner 98 is channel selected similarly as the 8VSB digital broadcasting wave, and is output as an IF signal. The IF signal outputted is amplified by the AMP 99 and is band limited by the Nyquist filter 100. The modulated signal which is output from the Nyquist filter 100 is then outputted to the orthogonal detector circuit 111. The orthogonal detector circuit 111 performs orthogonal detection for the carrier wave included in the modulated wave which is subjected to the band limitation with performing frequency locking and phase locking thereto, and outputs the result as a video signal to the LPF 112. Subsequently, the video signal having passed through the LPF 112 is output to the analog video processing circuit 113 to be processed thereby, and then displayed on a monitor or the like via the switch 122 and terminal 124.
The structure corresponding to the digital broadcasting receiver includes a tuner 114 for channel selecting a channel; a SAW filter 115 for applying a band limitation to the QAM modulated wave; an AMP 116 for amplifying the QAM modulated wave; a frequency converter 117 for converting the frequency of the QAM modulated wave; a LPF 118 for cutting the high frequency components of the frequency converted signal; a digital QAM demodulation circuit 119 for performing QAM demodulation processing; and a digital decoding and video processing circuit 120 for performing decoding and video processing to MPEG data.
The operation of the digital cable broadcasting receiver which performs the transmission by the QAM digital modulation will be described. Also in the case of QAM digital cable broadcasting, similarly as in the digital 8VSB broadcasting and the NTSC analogue broadcasting, the inputted RF signal is channel selected by the digital broadcasting tuner 114 and is outputted as an IF signal. Then, the signal is subjected to the frequency limitation which is required for the IF signal by the SAW filter 115, and is outputted to the AMP 116. The IF signal is then amplified by the AMP 116 and is outputted to the frequency converter 117. The QAM modulated signal, which is frequency converted to a further lower band than the IF band, is outputted, after the high frequency components thereof are cut by the LPF 118, to the digital QAM demodulation circuit 119 to be decoded thereby, and then is outputted as transport data to the next stage digital decoding and video processing circuit 120. The digital decoding and video processing circuit 120 performs MPEG decoding and video processing to the transport data to result an analogue, and this analogue signal is outputted through the switch 122 and terminal 124 to a monitor or the like to be displayed on a screen.
Heretofore, as the ground wave broadcasting, there has been provided analogue broadcasting such as NTSC. However, by the development of the digital transmission technology in recent years as described above, the digital ground wave broadcasting employing the digital modulation (particularly, 8VSB digital modulation in North America) has started. Also in the field of cable television, digital cable broadcasting service has started employing the conventional digital QAM modulation.
In the conventional constitution of multi-system corresponding receiver which can correspond to various transmission systems, the respective receivers have independent constitutions for the respective modulations systems, and even when considering a case where the ground wave broadcasting is received, it is necessary to switch between the receiver for receiving the analog broadcasting such as NTSC and the receiver for receiving the BVSB digital broadcasting. In addition, it is also necessary to switch the receiver for the digital cable broadcasting with the NTSC analog broadcasting or the digital ground wave broadcasting. It is very troublesome for the receiver's user to switch the receiver to that corresponding to the respective system at each time.
Further, in a system where the broadcasting of respective systems are seamlessly tuned by the same MPU, it takes a time for digital decoding processing and for decoding of data in the digital broadcasting, and there is a large difference in the time from the channel tuning to the video image being displayed on such as a monitor, as compared with at receiving the analog broadcasting. Accordingly, when the analog broadcast program and the digital broadcast program are seamlessly channel tuned, screen changes require a long period of time for the receiver's user in such as a case where a high speed channel up down operation is performed, thereby resulting a large inconvenience.